Electrochemical machining ferrous metals using a film forming electrolyte including fluoride salts



United States Patent 3,429,791 ELECTROCHEMICAL MACIHNING FERROUS METALS USING A FILM FORMING ELEC- TROLYTE INCLUDING FLUORIDE SALTS Mitchell A. La Boda, East Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Oct. 23, 1965, Ser. No. 504,096 U.S. Cl. 204-143 4 Claims Int. Cl. B23p 1/20, 1/16 ABSTRACT OF THE DISCLOSURE Electrolyte and coolant for the electrochemical machining of metals consisting essentially of an aqueous solution of an alkali metal chloride and an electrochemical erosion inhibiting film forming additive containing fluoride ions.

This invention relates to electrochemical machining processes and, more particularly, to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes, cavities and contoured surfaces have been developed and are more generally classified into one of two basic categories, the first being electrochemical machining and the second electrolytic grinding, a specialized application of the first. Electrolytic grinding is essentially an electrochemical deplating process which can be used on virtually any electrically conductive material. It is generally adapted to metal removal operations comparable to those performed by cutoff wheels, saws, and grinding or milling machines and the like, and uses equipment similar to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action.

A particular version of an electrolytic grinding process is characterized by a flow of electrolyte between the workpiece and a rotating grinding cathode wheel. The rotating cathode wheel comprises a conductive metal matrix having a plurality of nonconducting abrasive particles imbedded therein to provide nonconductive spacing between the workpiece and the cathodic matrix. Electric current is passed through the electrolyte to dissolve the anodic surfaces of the workpiece and the imbedded particles of the wheel abrade the surface to remove any irregularities resulting from nonuniform erosion or reaction product buildup.

While aqueous solutions of individual inorganic salts, such as nitrates, cyanides, carbonates, hydroxides and nitrites have been used as electrolytes in electrochemical machining and grinding processes, aqueous sodium chloride solutions appear to be best suited for general application work and are therefore most commonly used today. However, regardless of what salt is chosen, a major problem with electrochemical machining and grinding processes is overcut which is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. Such anodic dissolution can occur even in areas which are fairly well removed from the cathode. This overcut or cutting in low current density areas which are bathed in the electrolyte but substantially removed from the cathode, has been reduced in the prior art by the use of costly and time-consuming masking operations which isolate the areas to be machined by protecting the surrounding areas from the erosive elfect of the electrolyte. These masking operations are frequently quite involved and require a high degree of skill to insure a satisfactory product. Likewise, additional steps subse- 3,429,791 Patented Feb. 25, 1969 quent to the machining steps are required to strip the workpiece of the mask. Additionally, the prior art has attempted to reduce overcut by designing special purpose electrodes and machines to meet individual and specialized machining requirements.

By my invention I have at least reduced, and in most cases actually eliminated, the need for recourse to the prior arts attempted resolutions.

It is, therefore, an object of my invention to provide a self-masking electrolyte for ECM.

It is a further object of my invention to provide an additive for existing ECM electrolytes which selectively inhibits anodic dissolution in unwanted areas.

It is a further object of my invention to effect a sharply contoured machining by a process utilizing conventional electrolytes improved by additives which upon reaction with the workpiece form a film thereover, which film inhibits or stops-off electrolytic action or anodic dissolution in unwanted areas.

It is a further object of my invention to effect a sharply contoured electrochemical machining by a process utilizing aqueous electrolytes containing an additive consisting of a soluble inorganic fluoride ion containing compound.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

By my invention, 1 utilize the benefits derivable from the formation of insoluble reaction products on the surface of a workpiece. I have found that it is to my advantage not only to have the products formed, but to have them formed with improved adhesion and electrochemical erosion inhibiting film forming properties. I have found that when operating in accordance with my invention I can eifectively produce electrochemically machined parts having better dimensional tolerances than has heretofore been possible without recourse to the expediencies of the prior art. To effect these more desirably machined configurations, I have found it necessary merely to abrade away my improved film in those select areas where machining is to continue, leaving the balance of the masking film undisturbed. A particularly effective embodiment of my invention involves the addition of fluoride ions to electrochemical machining electrolytes. When added to these electrolytes, the additives of my invention, and especially fluoride salts of the alkali metals, create a solution which effectively forms a heavy adherent film over the surface of the workpiece. The film retards or substantially eliminates electrochemical erosion in those areas protected by the film. In a particular application of my invention a chloride electrolytic grinding electrolyte is modified by adding thereto potassium fluoride. The film formed is subsequently abraded away in those areas where electrochemical machining is to continue, hence presenting a limited uninhibited surface to unrestricted electrochemical action.

My experience has been that using the additives of my invention, I can successfully machine metal samples ranging from the softer low carbon steels (SAE 1008) to the harder low alloy steels (SAE 8610). Equally acceptable results may be experienced for all ferrous alloys wherein the iron content is 50% or more.

While the preferred electrolyte solution ranges from 7.5-l2.1%, by weight, of fluoride ion (19.83l.9% KP), 13% by weight, of sodium chloride and the balance water, I have found that effective electrolytes can be compounded using from as low as 0.70%, by weight, of fluoride ion (1.7% KF) to as high as 13.60%, by weight, of fluoride ion (35.75% KF), the balance being a selected concentration of an aqueous sodium chloride solution wherein the concentration of the sodium chloride may vary from dilute to saturated. A 10% fluoride ion concentration is particularly effective. In this connection the lighter alkali metal (lithium, sodium and potassium) chlorides and fluorides are preferred because they produce relatively neutral pHs, do not plate out or have a deleterious effect upon the cathode, and represent a source of inexpensive material. While I prefer the use of alkali metal fluorides, other fluoride compounds such as HF and NH F are also useful as a source of fluoride ions. It is significant to note that at the higher concentrations of my invention, I find it desirable to mix the respective cations in order to avoid the problems associated with the common ion effect. While electrolytes dilute as to chloride ion are operative, as a practical matter there are no significant advantages to operating at the lower concentrations and, in fact, it is less desirable to do so when considering such factors as solution conductivity and the like.

While the exact nature of the film is not known, an analysis indicates that a hydrated form of ferrous fluoride (FeF -XH O) is most probably formed. The low solubility of ferrous fluoride strengthens this conclusion.

The electrolyte will be particularly effective up to 40 volts and current densities of 500 amperes per square inch. However it is likewise effective at even higher current densities if so desired. As a practical matter, however, I prefer to operate between 100 and 200 amperes per square inch.

Tests were conducted utilizing a system wherein steel tube samples were brought up to a rotating sintered bronze diamond impregnated wheel. A gravity feed system kept the samples at the face of the wheel at all times. The feed system was such that an adjustable weight provided the capability of varying the pressures at which the samples would engage the wheel. It was found that to properly evaluate the inhibitive effects of my additives a minimum workpiece-to-wheel pressure should be employed in order to reduce the mechanical cutting component of the abrasive wheel. A room temperature electrolyte was pumped at a pressure of 9 p.s.i. through a bore in the workpiece and into the gap between the cathode and the workpiece. This gap was held constant at 0.0025 inches by the spacer effect of the nonconductive diamond chips. A current density between 50 amperes per square inch and 200 amperes per square inch was selected and maintained essentially constant throughout each test by varying the electrolyte flow rate between 0.19 gal. per minute and 0.31 gal. per minute and the speed of the rotating wheel between 150 r.p.m. and 250 r.p.m. An essentially constant voltage of 10 volts was maintained Tube length decrease per unit time was used to determine metal removal rates. The tube ends were compared with those produced by electrochemically grinding similar samples under the same conditions with additive-free electrolytes. The above cited conditions such as electrode gap etc. while certainly interdependent, one on the other, do not appear to be exceptionally critical and therefore might conceivably be varied considerably with equally satisfactory results.

The following are some specific examples encompassed within the scope of my invention:

EXAMPLE 1 A11 electrolyte comprising 1.7%, by weight, of potassium fluoride (0.7% F-), 17.4%, by weight of sodium chloride and the balance water was used to machine a sample of an SAE 1018 alloy. The temperature of the electrolyte was maintained at 70 F. and a potential of 10 volts was applied. A current density of 90 amperes per square inch was maintained, resulting in a metal removal rate of 0.016 inch per minute. The sample exhibited a good inhibition when compared to a sample tested similarly but using NaCl alone. At 200 a.s.i. this same solution produced even stronger inhibition.

EXAMPLE 2 An electrolyte comprising 31.86%, by We gh of P tassium fluoride (12.1% F-), 12.1%, by weight, of sodium chloride and the balance water was used to machine a sample of an SAE 1018 alloy. The temperature of the electrolyte was maintained at 70 F. and the potential at 10 volts. A current density of 150 amperes per square inch was maintained, resulting in a metal removal rate of 0.016 inch per minute. The sample exhibited an excellent squarely cut machining with a slight roughening of the surface finish. Additional samples tested at 50, and 250 a.s.i. respectively showed that the surface roughness varied directly with the increase in current density, but that the excellent character of the cut was retained.

EXAMPLE 3 An electrolyte comprising 35.75%, by weight, of potassium fluoride (13.55% F-), 11.4%, by weight, sodium chloride and the balance water was used to machine a sample of an SAE 1018 alloy. The temperature of the electrolyte was maintained at 70 F. and the potential at 10 volts. A current density of 100 amperes per square inch was maintained, resulting in a metal removal rate of 0.010 inch per minute. The sample exhibited an excellent square cut but presented a roughened finish.

I claim:

1. The process for electrochemically machining a ferrous metal and alloys thereof comprising the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathodic electrode adjacent to but closely spaced from said metal so as to form a gap therebetween, flowing through said gap a relatively neutral aqueous electrochemical erosion inhibiting film forming electrolyte consisting essentially of at least one alkali metal chloride and an additive supplying fluoride ion in a concentration of from about 0.7 percent to about 13.6 percent by weight to form said film, passing electric current through said metal, electrolyte, and electrode, and removing from selected areas the electrochemical erosion inhibiting film formed whereby electrochemcial machining can continue in said areas.

2. The process as defined in claim 1 wherein the concentration of said fluoride ion is from about 7.5% to about 12.1% by Weight.

3. In a process for electrochemically machining a ferrous metal and alloys thereof comprising the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathodic electrode adjacent to but closely spaced from said metal, flowing a relatively neutral aqueous electrochemical erosion inhibiting film forming electrolyte consisting essentially of at least one alkali metal chloride through said space and passing an electric current through said metal, electrolyte and electrode, the improvement comprising adding at least one additive supplying fluoride ions in a concentration of from about 7.5 percent to about 12.1 percent by weight so said solution to form said film and removing from selected areas the electrochemical erosion inhibiting film formed whereby electrochemical machining can continue in said areas.

4. The process as defined in claim 1 wherein the source of said fluoride ions is an alkali metal fluoride.

References Cited UNITED STATES PATENTS 3,088,889 5/1963 La Boda et a1. 3,130,138 4/1964 Faust et a1. 3,355,369 11/1967 Chaperon et al.

OTHER REFERENCES G. Keeleric et al.: Report No. MAB-18-M submitted under contract DA-49-025SC83 between Dept. of Defense and the Nat. Acad. of Sciences, J an. 18, 1952.

ROBERT :K. MIHALEK, Primary Examiner.

US. Cl. X.R. 204-66; 25279.3 

